1. Field of the Inventions
The present invention relates to manufacturing solar cell absorbers and, more particularly, manufacturing solar cell absorbers using electrodeposition processes.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (copper (Cu), silver (Ag), gold (Au)), Group IIIA (boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl)) and Group VIA (oxygen (O), sulfur (S), selenium (Se), tellurium (Te), polonium (Po)) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1−xGax (SySe1−y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a base 20 including a substrate 11 and a conductive layer 13. The substrate can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over the conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers comprising molybdenum (Mo), tantalum (Ta), tungsten (W), titanium (Ti), and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a cadmium sulfide (CdS), zinc oxide (ZnO) or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1. It should be noted that although the chemical formula for a CIGS(S) layer is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity, the value of k will be used as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The first technique that yielded high-quality Cu(In,Ga)Se2 films for solar cell fabrication was co-evaporation of Cu, In, Ga and Se onto a heated substrate in a vacuum chamber. Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin layers of Cu and In may be first deposited on a substrate and then this stacked precursor layer may be reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, for example use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CulnSe2 growth, for example, Cu and In layers were sequentially sputter-deposited on a substrate and then the stacked film was heated in the presence of a gas containing Se at elevated temperature for times typically longer than about 30 minutes, as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy layer and an In layer to form a Cu—Ga/In stack on a metallic back electrode layer and then reacting this precursor stack film with one of Se and S to form the absorber layer. Such techniques may yield good quality absorber layers and efficient solar cells, however, they suffer from the high cost of capital equipment, and relatively slow rate of production.
One prior art method described in U.S. Pat. No. 4,581,108 utilizes a low cost electrodeposition approach for metallic precursor preparation for a two-stage processing technique. In this method a Cu layer is first electrodeposited on a substrate. This is then followed by electrodeposition of an In layer forming a Cu/In stack during the first stage of the process. In the second stage of the process, the electrodeposited Cu/In stack is heated in a reactive atmosphere containing Se forming a CuInSe2 compound layer.
In another approach Cu—In or Cu—In—Ga alloys have been electroplated to form metallic precursor layers and then these precursor layers have been reacted with a Group VIA material to form CIGS type semiconductor layers. Some researchers electrodeposited all the components of the Group IBIIIAVIA compound layer. For example, for CIGS film growth electrolytes comprising Cu, In, Ga and Se were used. We will now review some of the work in this field.
Bonnet et al. (U.S. Pat. No. 5,275,714) electroplated Cu—In alloy layers out of acidic electrolytes that contained a suspension of fine Se particles. As described by Bonnet et al., this method yielded an electrodeposited Cu—In alloy layer which contained dispersed selenium particles since during electrodeposition of Cu and In, the Se particles near the surface of the cathode got physically trapped in the growing layer. Lokhande and Hodes (Solar Cells, vol.21, 1987, p. 215) electroplated Cu—In alloy precursor layers for solar cell applications. Hodes et al. (Thin Solid Films, vol.128, 1985, p.93) electrodeposited Cu—In alloy films to react them with sulfur to form copper indium sulfide compound layers. They also experimented with an electrolyte containing Cu, In and S to form a Cu—In—S layer. Herrero and Ortega (Solar Energy Materials, vol. 20, 1990, p. 53) produced copper indium sulfide layers through H2S-sulfidation of electroplated Cu—In films. Kumar et al (Semiconductor Science and Technology, vol.6, 1991, p. 940, and also Solar Energy Materials and Solar Cells, vol.) formed a Cu—In/Se precursor stack by evaporating Se on top of an electroplated Cu—In film and then further processed the stack by rapid thermal annealing. Prosini et al (Thin Solid Films, vol.288, 1996, p. 90, and also in Thin Solid Films, vol.298, 1997, p. 191) electroplated Cu—In alloys out of electrolytes with a pH value of about 3.35-3.5. Ishizaki et al (Materials Transactions, JIM, vol.40, 1999, p. 867) electroplated Cu—In alloy films and studied the effect of citric acid in the solution. Ganchev et al. (Thin Solid Films, vol.511-512, 2006, p. 325, and also in Thin Solid Films, vol.516, 2008, p. 5948) electrodeposited Cu—In—Ga alloy precursor layers out of electrolytes with pH values of around 5 and converted them into CIGS compound films by selenizing in a quartz tube.
Some researchers co-electrodeposited Cu, In and Se to form CIS or CuInSe2 ternary compound layers. Others attempted to form CIGS or Cu(In,Ga)Se2 quaternary compound layers by co-electroplating Cu, In, Ga and Se. Gallium addition in the quaternary layers was very challenging in the latter attempts. Singh et al (J. Phys.D: Appl. Phys., vol.19, 1986, p. 1299) electrodeposited Cu—In—Se and determined that a low pH value of 1 was best for compositional control. Pottier and Maurin (J. Applied Electrochemistry, vol.19, 1989, p. 361 electroplated Cu—In—Se ternary out of electrolytes with pH values between 1.5 and 4.5. Ganchev and Kochev (Solar Energy Matl. and Solar Cells, vol.31, 1993, p. 163) carried out Cu—In—Se plating at a maximum pH value of 4.6. Kampman et al (Progress in Photovoltaics, vol. 7, 1999, p. 1999) described a CIS plating method. Other CIS and CIGS electrodeposition efforts include work by Bhattacharya et al (U.S. Pat. Nos. 5,730,852, 5,804,054, 5,871,630, 5,976,614, and U.S. Pat. No. 7,297,868), Jost et al (Solar Energy Matl. and Solar Cells, vol.91, 2007, p. 636) and Kampmann et al (Thin Solid Films, vol.361-362, 2000, p. 309).
The above mentioned electrodeposition solutions employed for Cu—In, Cu—In—Ga, Cu—In—S, Cu—In—Se and Cu—In—Ga—Se film depositions do not yield stable and repeatable electrodeposition process and high quality films that can be used in electronic device applications such as solar cell applications. Therefore, there is a need to develop efficient electrodeposition solutions and methods to deposit smooth and defect-free Group IB-Group IIIA alloy or mixture films in a repeatable manner with controlled composition.